Dr Denman was willing to basically kidnap three girls to experiment on them, presumably never telling their families. And what was she going to do with Lewis, who would out her as soon as he could?
After Charlotte takes the locket and breaks Cleo's spirit - she swims out into a known shark breeding ground. Think about it: Cleo came close to committing suicide or at least some form of self-harm.
Fridge Logic:
So how do they avoid being seen in the school loo? Do they just not go in public?
In the first few episodes, Lewis tests out what will and won't turn them. Wipes won't, so presumably after using the toilet they use wipes instead of washing their hands. Possibly they also go to the bathroom in a group, which would be seen as normal among girlfriends, to keep an eye out in case there's an exposure risk.
"The Siren Effect" - Cleo says it's a Wednesday. So why is she having a sleepover on a school night? And why aren't the girls getting ready for school the next day?
Given that it was going to be a full moon that night, they probably thought it was just better to be around each other in case any of them ended up falling under the Moon Spell which, sure enough, Cleo ended up falling under.
It could be half term or a holiday. None of the adults the next morning says anything about going to school, and every boy in the area is camped out on Cleo's lawn.
